a!documentviewerfield not rendering PDF type files when clicked.

This looks like an unusual issue. Are you getting any error in the std-out.log file?

How will that be helpful in this situation ?

We only know this aftter reading the file. Could be some issue with the user's context. Or something that Appian isn't able to show on the front end. 

Also, here are some point you might want to confirm. 

1. Dev team is trying to access the file on the same environment as the end users. 
2. The document ID you are trying to pass to the viewer is a valid document ID
3. The PDF is a valid PDF and you have access to it, plus you are able to open it if you download. 

Thank you for the answer will look into the std-out.log file for more insight, please find my answer in red

1. Dev team is trying to access the file on the same environment as the end users.- Yes tried it on client environment
2. The document ID you are trying to pass to the viewer is a valid document ID - Yes
3. The PDF is a valid PDF and you have access to it, plus you are able to open it if you download.- Once downloaded the PDF is opening as expected

Thank you for the answer will look into the std-out.log file for more insight, please find my answer in red

1. Dev team is trying to access the file on the same environment as the end users.- Yes tried it on client environment
2. The document ID you are trying to pass to the viewer is a valid document ID - Yes
3. The PDF is a valid PDF and you have access to it, plus you are able to open it if you download.- Once downloaded the PDF is opening as expected